Where can we live??

[LordQulex]

I'm currently trying to build a near-future human timeline game in the hero system, trying to keep rubber science to a minimum. I'm in the process of creating solar systems for the 50 closest stars to Sol (a huge undertaking, if any of you are interested I wrote a planet generation program to speed up the process). The problem I'm running into is determining whether a human can survive unassisted on the planets surface. I was scouring Star Hero book and saw the -20 to 50 degree range developes life, but I didn't see anywhere about more conditions.

 

The only factors I can think of are atmosphere, gravity, and temperature. If those aren't right we can't live there. The range of the temperature is in the book, and it suggests people can't live on planets with more than 1.5x their home planet's gravity. But with all these rules about trace, thin, standard, and dense atmospheres, complex atmosphere composition formulae, and the hardest rotten table to program the atmospheric density table, I really can't figure out whether a person can realisticly breathe a planet's atmosphere.

 

Between web pages saying we need a certain percentage oxygen in the air, at a certain partial pressure, blah blah blah chemistry physics here... Can someone give me a rule of thumb like, "if the atmosphere is between W% and X% oxygen with no toxic elements, and the pressure is between .Y and 1.Z, people can survive there"?

 

If that isn't possible, can you give me some tips on how you determined whether people could breathe the atmo or not?

 

Thanks in advance!

[Nyrath]

Re: Where can we live??

 

from http://www.eldacur.com/~brons/NerdCorner/StarGen/StarGen.html

 

The definition of "Unbreathably thin atmosphere" and "Unbreathably thick atmosphere" are based on Dole. He gives 72mm of Mercury as the minimal inspired partial pressure of Oxygen. I've therefore used it as an absolute minimum total pressure in the descriptions. He gives 8 atmospheres as the pressure at which turbulence makes it impossible to inhale.

 

The rest of these labels are arbitrary and I'll happily change them if anyone can suggest better values.

 

In order to evaluate the breathability of the atmosphere, I took a formula from Dole to calculate the inspired partial pressure for a gas. I used this to compare each of the gases from Keris's model to the maximums and minimums listed in Dole. This allows me to categorize a planet's atmosphere as breathable, unbreathable (too little oxygen) or poisonous.

 

Dole's HABITABLE PLANETS FOR MAN is a bit dated. However, it is available as a free download here:

http://www.rand.org/pubs/commercial_books/CB179-1/

 

The current recommended book is World Building by Stephen L. Gillett

http://search.barnesandnoble.com/booksearch/isbninquiry.asp?ISBN=158297134X

[Cancer]

Re: Where can we live??

 

Poking around a bit, looks like oxygen toxicity sets in at 0.5 bar of oxygen pressure. Could well be lower than that; that's only 16 hours of exposure at 0.5 bar that starts causing symptoms in some folks.

 

Toxicity with respect to CO2 content is listed in volume fraction rather than pressures, mostly because O2 content matter is of strong interest in diving where the pressure varies substantially, while CO2 in large amounts is encountered under normal pressure conditions. At 1% CO2 you get drowsy, 2% there's mild impairment, 5% is acute, and 8% you pass out in 5 to 10 minutes. So the atmospheric CO2 content has to be rather low, no more than 1%. My guess is that really just means you need lots of liquid water on the surface of the planet; CO2 dissolves readily in water and an active atmospheric water cycle will scrub the CO2 out efficiently.

 

The only nontoxic buffer gases (gases that bring the total gas pressure up to the ~1 bar needed) that I know about are N2 and the inert gases. A terrestrial planet won't retain helium in the atmosphere so no need to consider that one. Neon and (in particular) argon may do it; I haven't looked into whether you'd do OK with a 20% O2 - 80% Ar mix at one bar of pressure. Argon is not all that common in terms of absolute elemental abundance, but it accumulates on planets because Ar-40 is a decay product of potassium 40, which is chemically reactive (so it gets included into rocks), has a long half-life (~10^9 years, so the potassium is incorporated into planet formation, which argon is not), and reasonably abundant.

 

Other cosmogenically abundant gases could be hydrogen (which, like helium, won't be retained by a terrestrial planet) and the compounds of H, C, N, and O with each other.

 

Methane oxidizes so rapidly that you don't have to worry about it if the O2 content is acceptable to you otherwise. Carbon monoxide is toxic but also reacts strongly enough with O2 that it shouldn't be a concern as long as your O2 level is high enough. Ammonia is toxic but again gets oxidized to N2 and water if you have lots of O2 at hand. HCN is even more toxic. Nitrogen oxides are rather reactive and toxic. That sort of exhausts the list of candidates.

[Asperion]

Re: Where can we live??

 

Another concideration that must be concidered is the distance that the planet has from the parent star. If the planet is too close then the star's radiation will over-energize everything and cook the people (unles they have sufficient shielding). This happens around the point that Venus has in the Sol System and closer. On the flip side, if the planet gets too far away from the parent star, then the radiation will be too diffused to do any real good. In the Sol System, Mars and beyond is at this point. As the star size increases, this zone will incrrease and move futher out, but the larger the star the shorter its life is. Scientists have determined that a star that is of basically Sol's characteristics is the only one that can sustain human life, and (correct me if wrong) the closest that a Sol-type star exists is greater than 50 light years away.

[LordQulex]

Re: Where can we live??

 

I'm not sure that is the case. I know a lot of solar radiation has to do with how much the atmosphere filters or fails to filter out. We could live on a closer planet with a denser atmo, or a further one with a thinner atmo. Now plants on the other hand, I'm not sure what affect a blue or red sun would have on plants we eat and our food eats. Maybe I'll rig up some red or blue lights for my hydroponic farm and see how they do.

 

I don't think orbital distance is as critical as atmospheric conditions. I generated one planet orbiting Proxima Centauri. Based on the dice rolls it was able to support life to create a livable place for us, despite being .15 AU away from the sun, and having no daily rotation (the temperature was very comfortable despite). I actually did the, what I call "complicated math" to figure out the atmosphere's composition of various gasses at various altitudes. This "complicated math" is what I want to circumvent.

 

It's probably not all that complicated, but I'm no chemist/astrophysicist. I found a few internet sources that say we need a partial pressure of oxygen between 100 and 400 billibars to be able to breathe that atmo. This can be determined without leaving earth because of our low-points and high points. I.E., people climbing Mt. Everest need O2 tanks cause the PP of O2 is less that 100 millibars, and deep-ocean divers need a special gas mix with less O2 to breathe that deep under water because the pressure compresses the gas up to the ~200 millibar level.

 

Also a caveat - I was doing some research about the human condition in high and low pressure areas. It seems that through nothing more than complete accidents, people have suck their hands into vacuums and been so deep under water to drill for oil to provide data that the human body can indeed exist in a near vacuum and over 33 atmosphere's worth of pressure with no permanent injury! So I really think that this partial pressure really is what determines whether an atmosphere is breatheable, cause our bodies will adapt to nearly any planetary surface pressure.

[LordQulex]

Re: Where can we live??

 

Nyrath-

 

Thanks a lot mate! I downloaded that PDF and it helped me develop an algorithm for what type of atmospheres we can breather! The key is we need between a partial pressure of O2 between 60 and 400 torr, or 80 to 533 millibar. Since 1 atmosphere ~= 1 bar, the table that gives atmospheric pressure as a product of atmospheric density and a planet's gravity can be used to calculate the partial pressure of oxygen. Take the table results, multiply it by the percentage of oxygen, and you have the partial pressure of oxygen. Along with the rest of my research I've got a good idea of how to determine whether an atmosphere is breathable or not! I think that we could survive anywhere from .5-10 bar comfortably.

[novi]

Re: Where can we live??

 

Another concideration that must be concidered is the distance that the planet has from the parent star. If the planet is too close then the star's radiation will over-energize everything and cook the people (unles they have sufficient shielding). This happens around the point that Venus has in the Sol System and closer. On the flip side' date=' if the planet gets too far away from the parent star, then the radiation will be too diffused to do any real good. In the Sol System, Mars and beyond is at this point. As the star size increases, this zone will incrrease and move futher out, but the larger the star the shorter its life is. Scientists have determined that a star that is of basically Sol's characteristics is the only one that can sustain human life, and (correct me if wrong) the closest that a Sol-type star exists is greater than 50 light years away.[/quote']

 

Actually, the nearest sun-like star is our next door neighbor, Alpha Centauri.

 

And, because other people have done the hard work for me:

 

http://en.wikipedia.org/wiki/Solar_analog

 

Also, habitability and alien life are two very different things. Intelligent alien life makes it like 4 different things. And all scientist have determined on the subject is that we evolved around Sol, and most of that on Earth. Anything else is speculation, since you can't do good science with only one data point.

[novi]

Re: Where can we live??

 

Gotta agree with Cancer on atmospheres. Realistically, nitrogen will be your major non-oxygen component of the atmosphere, but you can get away with majority argon atmospheres. You'd need some nitrogen around for an ecosystem, but I'm guessing that you only need about as much nitrogen in the atmosphere as there is CO2 in Earth's atmosphere. Neon is not likely to be there - despite it's overall abundance in the universe, it either doesn't tend to accumulate in planets as they form and/or is light enough to get blown out of the atmosphere like helium.

 

The other question I have is are you looking for already habitable planets, or for easily terraformable planets? The former requires life to have already evolved on them, since that is the only reasonable way to get enough oxygen into the atmosphere. It will also take 2 to 5 billion years (Earth=4) to get it up to a breathable level naturally. The latter just requires that temperature and gravity are right, and that we'll take care of atmospheric composition.

 

Gravity is the easiest one. Max gravity of 1.5 g's, minimum is less obvious, let's rig the scale so that Mars counts and peg it at .35 g's. That's probably about as low as you can go and expect to hold a decent atmosphere on a usable timeframe. For an already habitable planet, it needs to be a bit bigger so that it remains tectonically active. Again guessing, but probably needs to be at least half Earth mass to stay active for 4 billion years.

 

Temperature is more complicated, since it has a quantity and quality portion. The quality portion requires that the star produces the right colors of light in sufficient levels to do photosynthesis. On Earth, that requires normal visible light. It may be possible to run photosynthetic chemistry off infrared light, but it seems fairly unlikely. It would be even easier to run photosynthesis off ultraviolet, but you run into issues with ozone levels and skin cancer. The quantity portion just requires that the there be enough light.

 

In practical terms for temperature, the table in Star HERO p77 is decent, though my understanding is that the green zone is a bit further out than the table lists, and also depends on the mass of the planet in question. Namely, our G2V star is thought to have a green zone from .9-1.6 AU, as making a planet warmer is a lot easier than making a planet colder. Also, for quality, suitable stars are all Gs, hotter Ks, and cooler Fs.

 

One last note, before I get to sleepy to say anything more, is that terraforming a planet takes on the order of a thousand years. So, depending on just how hard your sci-fi is, you can fudge that by up to an order of magnitude without too much effect. So you can have planets mostly terraformed in just 100-200 years, though they may still have interesting chemistry as minerals are not yet fully oxidized.

[Nyrath]

Re: Where can we live??

 

Also' date=' habitability and alien life are two very different things.[/quote']

Also note that "habitable" means "Human habitable".

Earth is the ultimate example of human habitable, but it would be very uninhabitable to the organic hydrogen dirigibles of Jupiter or the living lava of Mercury.

[assault]

Re: Where can we live??

 

The other question is: is a world with locally evolved life human habitable?

 

I'm not talking about sentient aliens, or even multi-cellular non-terrestrial life - just plain old non-terrestrial life.

 

There are both practical and ethical questions here:

* is it possible to live on a world with native non-terrestrial life?

* if not, is it possible to make it possible to live there?

* is it ethical to do the latter, given the likely impact on the local ecosystems?

* is it a good idea to destroy alien ecosystems that you know nothing about?

 

Personally, I think Mars is pretty much an optimal world for humans to settle... Unfortunately there aren't a lot of good economic reasons to do so.

[novi]

Re: Where can we live??

 

Also note that "habitable" means "Human habitable".

Earth is the ultimate example of human habitable, but it would be very uninhabitable to the organic hydrogen dirigibles of Jupiter or the living lava of Mercury.

 

Yeah, I was trying to point out that the set of planets that have a natural breathable atmosphere is a subset of the planets that exist in a star's habitable zone. Our solar system currently has one planet with a natural, breathable atmosphere, but arguably two in it's habitable zone. And a billion years ago, it would have had zero planets with a natural, breathable atmosphere. But for the last four billion years, it would have had two planets that could be easily terraformed to have a breathable atmosphere (alas, Venus has always been just a bit too hot to salvage easily).

 

But, to get back to your original question, which I haven't strictly answered, based on the table from the Star HERO book, only planets with thin and standard atmospheres will have breathable atmospheres. A pressure range of .35-3 is your rough limit, limited on the low end by the minimum partial pressure of oxygen, and the upper end by gas narcosis and oxygen toxicity.

 

http://en.wikipedia.org/wiki/Nitrogen_narcosis

 

More specifically, as p87 lays out, the randomly generated oxygen level is 1d6 * 5%, and you halve the originally generated pressure. After you calculate the pressure from the gravity and density, multiply that by the %oxygen, and any result between .1 and .4 atmospheres is breathable.

 

Furthermore, his method of generating random atmospheres is crap, mostly since by his rules, you can't generate a copy of earth. Either skip the halving step, or double the primordial value.

 

Toxicity is fairly simple, since all toxic gases are highly reactive with either oxygen or water, and will get cleaned out of the atmosphere, long term. You still have to watch out for excessive volcanism.

 

Oh, and for a planet to have free oxygen in the atmosphere, it must have native, photosynthetic life. The likelihood of having life is a complete guess, with current theories all the way from improbable to certainty. My understanding is that simple life, aka bacteria, is probably common, and possibly even a virtual certainty on planets with liquid water. Developing photosynthesis is another unknown chance, but is highly favored by natural selection if it does happen.

 

So figure out how likely it is for a planet to have life, and establish a time to convert the atmosphere. The book suggest a flat 3 billion years, maybe randomize that a bit, and any planet older than that will have a breathable atmosphere.

[wcw43921]

Re: Where can we live??

 

I would think that gravity is the biggest of all these considerations. Lack of atmosphere or temperature variations can be dealt with throught the use of artificial habitats--sealed underground caverns or surface structures that are properly shielded and pressurized to protect and sustain human life. Gravity is the only thing for which human science as we know it cannot compensate, unless you state that artificial gravity exists--which would contradict your statement about keeping rubber science to a minimum. And without something like A-G to provide a gravitic "standard," people who have become accustomed to living and working with a specific gravity would have difficulty adjusting to a world with a different gravity, especially if it were greater than that of his homeworld.

 

Unless you decide that there are plenty of worlds with gravity similiar to Earth's, it may be necessary for your PCs to build O'Neill-style space colonies like Babylon 5 in other star systems, rather than colonizing the planets themselves.

 

(This idea comes from an Isaac Asimov story I read many years ago, in which the child of lunar colonists, having been born and raised on the Moon, could not visit Earth--understandable, as six times the gravity would be uncomfortable for anybody, to say the least.)

 

Hope that helps.

[Asperion]

Re: Where can we live??

 

The other question is: is a world with locally evolved life human habitable?

 

I'm not talking about sentient aliens, or even multi-cellular non-terrestrial life - just plain old non-terrestrial life.

 

The answer to this actually lies in how close both the planet itself and life forms are to human life. My belief is that to accomplish this the planet would be so similar to Earth that it is not realistically possible without major atmospheric change, that would destroy the native life found there.

 

There are both practical and ethical questions here:

* is it possible to live on a world with native non-terrestrial life?

* if not, is it possible to make it possible to live there?

* is it ethical to do the latter, given the likely impact on the local ecosystems?

* is it a good idea to destroy alien ecosystems that you know nothing about?

 

Personally, I think Mars is pretty much an optimal world for humans to settle... Unfortunately there aren't a lot of good economic reasons to do so.

 

Humans could live on that world if the planetary conditions were similar to Earth, but that would be unlikely. We could make the planet human livable, but in the process the native life will be destroyed. As for the ethical considerations, that could only be determined by the GM. If the GM wants a game similar to the original Star Trek, then have the humans go around forcing all others to adopt human values. Or the GM could say a society that is similar to Next Gen Star Trek. In this case the society will do everything in their power to preserve all life in its original form and will not do anything other than small self contained bubbles on such planets. It is never a good idea to destroy the native life since they could teach us many things about life in general and give everyone much in the way of technology, medicine, chemistry, and every other aspect of human life.

[Nyrath]

Re: Where can we live??

 

http://crowlspace.com/?p=365